Electric welder with current-voltage feedback circuit that provides desired slope curve

ABSTRACT

A D.C. welder includes an engine-driven D.C. generator, a  polarity-revers switch which can change the direction of current flow through the field winding of that generator to provide a positive or negative polarity at the welder output terminals, a circuit that automatically permits only uni-directional current flow through that field winding during the starting of the engine, and a further circuit that energizes a solenoid to close contacts which enable that generator to operate as the starting motor for that engine but which thereafter de-energizes that solenoid and then keeps it de-energized until the engine is at, or close to, rest.

This application is a division of application Ser. No. 944,962, filedSept. 22, 1978 now U.S. Pat. No. 4,293,756.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a D.C. welder which has an engine-drivenD.C. generator. A polarity-reversing switch can change the direction ofcurrent flow through the field winding of that generator to provide apositive or negative polarity at the welder output terminals. A circuitautomatically permits only uni-directional current flow through thatfield winding during the starting of the engine, and thereby enablesthat generator to operate as the starting motor for that engine. It is,therefore, an object of the present invention to provide a D.C. welderwhich includes an engine-driven D.C. generator, a polarity-reversingswitch which can change the direction of current flow through the fieldwinding of that generator to provide a positive or negative polarity atthe welder output terminals, and a circuit that automatically permitsonly uni-directional current flow through that field winding during thestarting of the engine.

A solenoid can be energized to close contacts which will connect abattery to the armature of the generator to enable that generator tooperate as the starting motor of the engine during the starting of thatengine. A circuit automatically permits that solenoid to be energizedduring the starting of that engine, but positively de-energizes thatsolenoid, and then prevents energization of that solenoid, when thatengine reaches a predetermined speed. In doing so, that circuit preventsthe injury to that battery and also prevents the injury to thatgenerator which could occur if those contacts were closed to connectthat battery to that armature while that generator was providing weldingcurrent. It is, therefore, an object of the present invention to providea solenoid that can be energized to close contacts which will connect abattery to the armature of a generator to enable that generator tooperate as the starting motor of an engine during the starting of thatengine and to provide a circuit which automatically permits thatsolenoid to be energized during the starting of that engine butpositively de-energizes that solenoid, and then prevents energization ofthat solenoid, when that engine reaches a predetermined speed.

The welder has a circuit which controls the amount of current that canflow through the field winding of the generator. That circuit providesmaximum current flow through that field winding during the starting ofthe engine so the generator can provide maximum starting torque for thatengine; but that circuit becomes inactive after the engine reaches apre-set speed. Thereafter, further circuits can control the amount ofcurrent flowing through the field winding. It is, therefore, an objectof the present invention to provide a circuit which controls the amountof current that can flow through the field winding of the generator of awelder, which provides maximum current flow through that field windingduring the starting of the engine, and which becomes inactive after theengine reaches a pre-set speed.

Other and further objects and advantages of the present invention shouldbecome apparent from an examination of the drawing and accompanyingdescription.

In the drawing and accompanying description a preferred embodiment ofthe present invention is shown and described but it is to be understoodthat the drawing and accompanying description are for the purpose ofillustration only and do not limit the invention and that the inventionwill be defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, FIG. 1 is a perspective view of one preferred embodimentof electric welder that is made in accordance with the principles andteachings of the present invention,

FIG. 2 is a further perspective view of the electric welder of FIG. 1,but it shows that welder after it has been rotated one hundred andeighty degrees about a vertical axis,

FIG. 3 is a view, on a larger scale, of the control panel of theelectric welder of FIGS. 1 and 2,

FIG. 4 is an elevational view, on a scale intermediate those of FIGS. 1and 3, which shows some of the components of the electric welder after aside panel has been removed.

FIG. 5 is an elevational view, on the scale of FIG. 4, of the oppositeside of the electric welder of FIGS. 1 and 2 after the panel for thatside has been removed,

FIG. 6A is a schematic diagram of some of the components and connectionsof the electric welder of FIGS. 1 and 2,

FIG. 6B is a schematic diagram of further of the components andconnections of that electric welder, and

FIG. 6C is a schematic diagram of the rest of the components andconnections of that electric welder.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Mechanical andElectro-Mechanical Components

Referring to the drawing in detail, the numeral 10 generally denotes onepreferred embodiment of electric welder that is made in accordance withthe principles and teachings of the present invention. That welder has ahousing which has a front panel 12, a rear panel 14, side panels 16 and18, and a top panel 20. Those side panels have louvers and screenedopenings therein to permit ready ingress or egress of air; and the toppanel has a large unobstructed opening adjacent one end thereof. Apartition 22 and a partition 24 generally subdivide the interior of thehousing for the welder 10 into three compartments. A generator 50 and analternator 58 are disposed within the center compartment, an internalcombustion engine 56 and an alternator 60 are disposed in one of the endcompartments, and a fuel tank 25 and a storage battery 62 are located inthe other end compartment. The partition 24 has vertically-directedslots, not shown, therein to accommodate belts which extend between thepulleys on the end of the crank shaft of engine 56 and pulleys on theends of the armatures of generator 50 and of alternator 58. Thepartition 22 is substantially devoid of openings that could permit airto pass freely between the compartments which are separated by thatpartition.

The engine 56 has a muffler 40, an exhaust pipe 42, and a movablerain-excluding cover 44. As shown by FIGS. 4 and 5, that exhaust pipeand that rain-excluding cover extend upwardly through the large openingin the top panel 20. The air filter for engine 56 is denoted by thenumeral 45; and it is located in the center compartment at a point abovethe level of alternator 58. The air duct 46 extends from the air intakeof engine 56 to and through the partition 24; and a flexible air duct 47extends from the end of air duct 46 to the outlet of air filter 45. Theinlet 48 of that air filter extends through partition 22 adjacent thetop of that partition to communicate with the atmosphere above the fueltank 25.

During the operation of the engine 56, the air which is drawn into theair intake of that engine will pass through the inlet 48 of air cleaner45, through that air cleaner, through flexible air duct 47, and thenthrough the air duct 46 to that air intake. Because the air inlet 48communicates with the compartment in which the fuel tank 25 and battery62 are located, four important benefits are attained. First, anyvaporized fuel from that fuel tank will be drawn away from that fueltank and out of the fuel tank compartment to engine 56--with consequentutilization of that fuel vapor, and also with consequent minimization ofthe risk of fire due to fuel vapor in that compartment. Second, anycombustible gas, such as hydrogen, from the battery 62 will be drawnaway from that battery and out of the fuel tank compartment to engine56--with consequent utilization of that gas, and also with consequentminimization of the risk of fire due to combustible gas in thatcompartment. Third, the air within the fuel tank compartment usuallywill be freer of dust than the ambient air. Fourth, the drawing of airfrom the fuel tank compartment will tend, on hot days, to reduce thetemperature within that compartment, and hence will tend to minimizevaporization of the fuel within fuel tank 25. As a result, the welderprovided by the present invention can be operated more efficiently onhot days--even when it is mounted in the back of a pickup truck, andhence is exposed to direct rays from the sun.

GENERATOR AND ALTERNATORS

Referring to the drawing in detail, power ground is denoted by atriangle and signal ground is denoted by several horizontal lines whichgenerally define a triangle. The generator 50 preferably is a D.C.generator which is produced by Teledyne Waterboro and which issubstantially identical to a prior D.C. generator that TeledyneWaterboro has manufactured and sold for a number of years under thedesignation Teledyne 676050 or Ordnance Part Number 10898795. Thesignificant difference between generator 50 and that prior generator isthe mounting of the brushes--the brushes in generator 50 beingcircumferentially adjustable whereas the brushes in that prior generatorare fixed. Less significant differences between generator 50 and thatprior generator are differences in the end bells and in the fans, andthe use of differing number of bolts. The armature of generator 50 isdenoted by the numeral 52 in FIG. 6B; and the field winding of thatgenerator is denoted by the numeral 54.

The pulleys and belts which connect the crankshaft of engine 56 to thearmature 52 enable that armature to drive, and to be driven by, thatcrankshaft. Further pulleys and a further belt enable that crankshaft todrive alternator 58 and alternator 60. Although different engines couldbe used as the engine 56, a Teledyne Wisconsin VH4D aircooled engine isvery useful. The alternator 58 is used to supply power for polishingtools and the like; and it will preferably be a standard one hundred andfifteen volt A.C., sixty Hertz, two kilowatt or three kilowattalternator. The alternator 60, and the rectifier therein, are used tocharge the battery 62; and it will preferably be a standard,rectifier-equipped automotive alternator that provides twelve volts D.C.The battery 62 can supply power to armature 52 and to field winding 54to enable generator 50 to serve as the starting motor for engine 56.

The numerals 64, 66 and 68 in FIG. 6C denote the output terminals of thewelder provided by the present invention. A conductor 69 and a portion73 of the tapped primary winding of a rate transformer 70 in FIG. 6Cconnect generator brush 53, which bears against the commutator ofarmature 52, to output terminal 66; and that conductor and that portionplus the other portion 74 of that tapped primary winding connect thatbrush to output terminal 64. A low resistance "shunt" 72 in FIG. 6B, fora D.C. ammeter 75, connects generator brush 55, which also bears againstthe commutator of armature 52, to output terminal 68 via a conductor 67.If desired, the output terminal 64 and the portion 74 of the tappedprimary winding 70 could be eliminated; and, in that event, all valuesof welding current would be supplied by output terminals 66 and 68. AD.C. voltmeter 71 is connected to output terminals 66 and 68, as shownby FIG. 6C.

The alternator 58 has a field winding and a solid state rectifierregulator which can be connected to, or disconnected from, the duplexoutlet receptacle 32 in FIG. 3 by a switch 30. When that switch is"open", that field winding will be de-energized, and hence thatalternator will be unable to generate any power. When that switch isclosed, that field winding can be energized or self-excited by thealternator output through that solid state rectifier regulator; and thatsolid state rectifier regulator will vary the field winding current tomaintain the desired output voltage. A fuse 34 is connected between theoutput of alternator 58 and the duplex outlet receptacle 32. A lamp 36is normally dark but will become illumined to indicate when thatalternator is supplying power to that duplex outlet receptacle.

ENGINE-STARTING SYSTEM

As shown by FIG. 6B, the positive terminal of battery 62 is connected toone contact 76 of a solenoid switch of the type used in engine-startingcircuits of trucks and automobiles. The other contact 76 is connected tobrush 53 of generator 50; and hence, whenever the contacts 76 areclosed, current will flow from the positive terminal of battery 62through contacts 76, through armature 52, and through shunt 72 to powerground--and hence to the negative terminal of that battery. The coil ofthe solenoid switch is denoted by the numeral 78; and anenergy-dissipating diode 77 is connected in parallel with that solenoid.

The numeral 80 denotes a fuse which connects the positive terminal ofbattery 62 to a junction 81 at the input of a key-operated off-on-startswitch 82 of the type used in trucks and automobiles. A movable contact83 of that switch normally is in an "off" position, but it can be movedto a "start" position to cause current to flow from the positiveterminal of battery 62 via fuse 80, junction 81, that movable contact,the adjacent stationary "start" contact, solenoid coil 78, a junction84, and the collector-emitter circuit of an NPN transistor 86 to powerground. A diode 88 and a diode 90 are connected in series between powerground and junction 81 by a conductor 79; and the cathode of diode 88and the anode of diode 90 are connected to the collector of transistor86. Those diodes will protect that transistor against injury due toinductive transients. The transistor 86 is conductive, and hencesolenoid coil 78 can be energized, only during the starting of engine56--all as explained hereinafter in the Solenoid Disconnecting Circuitsection. The other movable contact 85 of switch 82 normally is in an"off" position, but it can be moved into engagement with the run-startcontact of that switch to connect junction 81 to the ignition system ofengine 56 and to other portions of the overall circuit for the welder bya conductor 87.

An NPN transistor 89 in FIG. 6C is connected between junction 81 andpower ground by conductor 79; and, similarly, a series-connected Zenerdiode 91 and a resistor 93 are connected between that junction and powerground by that conductor. The junction between Zener diode 91 andresistor 93 is connected to the base of transistor 89. That transistor,Zener diode and resistor constitute a clipping circuit which isconnected in parallel with battery 62; and it is intended to shunt toground any transients which appear at junction 81 and which have a valuein excess of twenty volts plus the base-emitter voltage drop oftransistor 89. Specifically, Zener diode 91 normally is non-conductive,and hence transistor 89 also is normally non-conductive. However, in theevent a transient develops at junction 81, which has a value greaterthan twenty volts plus the base-emitter voltage drop of transistor 89,Zener diode 91 will become conductive. The resulting flow of currentthrough the base-emitter circuit of transistor 89 will render thattransistor conductive; and, thereupon, the collector-emitter circuit ofthat transistor will pass that transient to power ground.

An NPN transistor 95, a resistor 97, a Zener diode 99, a capacitor 101,a diode 103 and a capacitor 109 constitute a limiter which is connectedbetween the run-start contact of switch 82 and power ground by conductor87 and a conductor 194. That limiter is intended to pass to power groundsubstantially all transients which appear at that contact. Specifically,resistor 97 forward biases transistor 95 to render that transistorconductive; and hence any transients which appear at the run-startcontact of switch 82 will tend to be passed to ground via conductors 87and 194 and the collector-emitter circuit of transistor 95 and then, inparallel relation, via capacitor 109 and also diode 103 and capacitor101. In the event any such transient had a value greater than fifteenvolts, the normally non-conductive Zener diode 99 would becomeconductive; and, thereupon, resistor 97 and Zener diode 99, as well asthe collector-emitter circuit of transistor 95, capacitor 109, and diode103 and capacitor 101, would pass that transient to power ground. As aresult the emitter of transistor 95 can supply twelve volts D.C., whichis effectively free of transients, to the input of a regulator 92 and,via a junction 105 and a branched conductor 107, to various points inthe overall welder circuit.

The regulator 92 has the "ground" terminal thereof connected to signalground, and has the output thereof spaced from ground by a capacitor 94.That regulator will provide a regulated eight volts D.C. to a junction111 adjacent its output and, via a conductor 113, to various points inthe overall welder circuit.

SOLENOID DISCONNECTING CIRCUIT

At "start up" of the welding system, the transistor 86 in FIG. 6B willbe forward biased by the Solenoid Disconnecting Circuit which is shownin FIG. 6A and which includes a conductor 115, a resistor 96, andcapacitors 98 and 100 that can couple pulses, from the "breaker" pointsof the ignition system of engine 56, to the inverting input of a Nortonamplifier 102 while by-passing any high frequency transients to signalground. The output of that Norton amplifier is coupled to the invertinginput of a Norton amplifier 118 by a capacitor 106 and a resistor 108which has a diode 110 connected in parallel with it. The output ofNorton amplifier 118 is fed back to the inverting input of Nortonamplifier 102 by a diode 120 and a resistor 122; and that output also isintegrated by a resistor 124 and a capacitor 126. An adjustable resistor128, a conductor 129, and a resistor 130 connect the ungrounded terminalof capacitor 126 to the inverting input of a Norton amplifier 136 inFIG. 6C. A resistor 138 connects the output of Norton amplifier 136 tothe base of an NPN transistor 140 which has the emitter thereofconnected to the base of transistor 86. The remaining components of theSolenoid Disconnecting Circuit are resistor 104, 112, 116, 132, 134, 142and 144, capacitor 114, and a conductor 131. Conductor 113 in FIG. 6Aand the resistors 104, 116 and 132 connect the non-inverting inputs ofNorton amplifiers 102, 118 and 136 to the regulated eight volts D.C. atthe output of regulator 92 in FIG. 6C. Pins 14 of those Nortonamplifiers are connected directly to that regulated eight volts D.C. byconductor 113 and by connections, not shown; and pins 7 of those Nortonamplifiers are connected directly to signal ground. Conductor 113 andresistor 112 connect the inverting input of Norton amplifier 118directly to the regulated eight volts D.C.; and capacitor 114 isconnected between that input and signal ground. Resistor 134 connectsthe output of Norton amplifier 136 to the non-inverting input of thatNorton amplifier. Resistor 142 and conductor 107 connect the collectorof transistor 140 to the junction 105 via conductor 107 and hence toessentially transient-free twelve volts D.C.; and resistor 144 connectsthe emitter of that transistor to power ground.

When engine 56 is at rest, the "breaker" points of the ignition systemof that engine wil not develop, and hence will not supply, pulses forthe Solenoid Disconnecting Circuit. However, when the crankshaft of thatengine is rotating, those "breaker" points will supply pulses toresistor 96 in FIG. 6A via conductor 115. Capacitor 100 will couplethose pulses to a one shot multivibrator which includes the Nortonamplifiers 102 and 118, resistors 104, 108, 112, 116 and 122, capacitors106 and 114, and diodes 110 and 120. That multivibrator will respond toeach pulse from the "breaker" points to provide an output pulse whichhas a predetermined amplitude and a predetermined width; and thatmultivibrator will apply those output pulses to the integrator which isconstituted by resistor 124 and capacitor 126.

The regulated eight volts, which conductor 113, resistor 132 andconductor 131 apply to the non-inverting input of Norton amplifier 136in FIG. 6B, will tend to cause the output of that amplifier to be alogic "1". Consequently, until the value of the voltage at the invertinginput of that amplifier exceeds the value of the voltage at thatnon-inverting input, that output will be a logic "1". As the engine 56is being started, the one shut multivibrator will respond to the pulsesfrom the "breaker" points to apply output pulses to the integrator; andadjustable resistor 128, conductor 129, and resistor 130 will respond tothe consequent increase in voltage at the upper terminal of capacitor126 to apply a signal to the inverting input of Norton amplifier 136.When the movable contact of that adjustable resistor is at one end ofits path of adjustment, the engine crankshaft will have to rotate ateleven hundred or more revolutions per minute before the voltage at theinverting input of Norton amplifier 136 can exceed the value of thevoltage at the non-inverting input of that amplifier. However, when thatmovable contact is at the other end of its path of adjustment, theengine crankshaft need only rotate at two hundred revolutions per minuteto cause the voltage at the inverting input of Norton amplifier 136 toexceed the value of the voltage at the non-inverting input of thatamplifier. Usually that movable contact will be set so the voltage atthe inverting input of Norton amplifier 136 will exceed the value of thevoltage at the non-inverting input of that amplifier when the crankshaftreaches about three hundred revolutions per minute. Once the voltage atthe inverting input of Norton amplifier 136 exceeds the value of thevoltage at the non-inverting input of that amplifier, the output of thatamplifier will change from a logic "1" to a logic "0".

All of this means that whenever the engine 56 is at rest or is operatingat less than a predetermined speed, the Norton amplifier 136 will beapplying a logic "1" to the base of transistor 140 via resistor 138, tothe non-inverting input of a Norton amplifier 152 in FIG. 6C via aconductor 146, a diode 148 and a resistor 150, and to the base of an NPNtransistor 228 in FIG. 6A via a conductor 147 and a resistor 230.Transistors 140 and 228 will respond to that logic "1" to be conductiveat the saturation level; and Norton amplifier 152 will respond to thatlogic "1" to provide a logic "1" at the output thereof. The significanceof the conducting of transistor 228 at the saturation level and thedevelopment of the logic "1" at the output of Norton amplifier 152 willbe explained in subsequent sections. The conducting of transistor 140 atthe saturation level will cause transistor 86 to conduct at thesaturation level. Consequently, whenever the engine 56 is at rest or isoperating at less than a predetermined speed, transistor 86 will permitsolenoid coil 78 to be energized by the shifting of switch contact 83 tothe "start" position. Also whenever the engine 56 is at rest or isoperating at less than a predetermined speed, transistor 86 will permitcurrent to flow from the "run-start" contact of switch 82 via conductors87 and 194 in FIG. 6B, a relay coil 180, junction 84, and thecollector-emitter circuit of transistor 86 to power ground. Thesignificance of the energization of that relay coil will be explained ina subsequent section.

As soon as the crankshaft of the engine 56 reaches a speed at which thecapacitor 126, adjustable resistor 128 and resistor 130 in FIGS. 6A and6B make the voltage at the inverting input of Norton amplifier 136exceed the value of the voltage at the non-inverting input of thatamplifier, the output of that amplifier will change from a logic "1" toa logic "0". Thereupon the forward biases on transistors 140 and 228 inFIGS. 6A and 6B will disappear and both of those transistors will becomenon-conductive. The logic "0" at the output of Norton amplifier 136 willback-bias diode 148 in FIG. 6C; and hence the output of that amplifierwill no longer affect the non-inverting input of Norton amplifier 152 inFIG. 6C. As transistor 140 becomes non-conductive, the forward bias fortransistor 86 will disappear; and hence that transistor also will becomenon-conductive. At this time, solenoid coil 78 and relay coil 180 willbe isolated from power ground; and hence that relay coil will becomede-energized, and that solenoid coil will become de-energized if theswitch contact 83 is in the "start" position, and it will be keptde-energized if it was previously de-energized as that contact waspermitted to shift to the "run" position. Thereafter, as long as thecrankshaft rotates at a speed greater than the speed set by the movablecontact of adjustable resistor 128, the solenoid coil 78 and the relaycoil 180 will be kept de-energized, transistor 228 will be keptnon-conducting, and diode 148 will be kept back-biased. The action ofthe Solenoid Disconnecting Circuit in keeping solenoid coil 78de-energized while the generator 50 is providing welding power enablesthat circuit to act as a safety interlock citcuit. The continuedde-energization of solenoid coil 78 is important; because it willprevent the destruction of battery 62 and the injury to the generator 50which might occur if the contacts 76 were closed while that generatorwas developing welding power.

GENERATOR FIELD CIRCUIT

Current is supplied to the field winding 54 of generator 50 in one oftwo directions or polarities, as determined by the setting of apolarity-reversing switch 174 and by the positions of relay contacts 184and 190. Specifically, if switch 174 is in the "reverse" position ofFIG. 6B, current will flow from conductor 87 via contacts 162 and 164and conductor 199 to and through field winding 54 to provide a positivepolarity at brush 53--regardless of the positions of relay contacts 184and 190. However, if switch 174 is in its "straight" position, currentwill flow from conductor 87 via relay contacts 186 and 184, switchcontacts 166 and 164 to and through field winding 54 to provide apositive polarity at brush 53 if a relay coil 180 is energized, andcurrent will flow from conductor 87 via relay contacts 188 and 190,switch contacts 172 and 170, conductor 193, the secondary winding ofrate transformer 70, and conductor 197 to and through field winding 54if relay coil 180 is de-energized.

The current which will flow through field winding 54 is variouslysupplied by a sub-circuit which includes Norton amplifier 136 in FIG.6B, conductor 146, diode 148 in FIG. 6C, resistor 150, Norton amplifier152, a resistor 154, NPN transistors 156 and 160, resistors 158 and 162,a conductor 176, contacts of switch 174, conductor 193, the secondarywinding of rate transformer 70 in FIG. 6C, conductor 197, field winding54 in FIG. 6B, conductor 199, conductor 87 and the contacts controlledby relay coil 180. A manually-operable switch 178 in FIG. 6C isconnected in parallel with the secondary winding of rate transformer 70;and that switch is shown in its open position. Resistor 158 andconductor 87 connect the collector of transistor 156 to the "run-start"contact of switch 82 in FIG. 6B; and resistor 162 in FIG. 6C connectsthe emitter of that transistor to power ground. The emitter oftransistor 160 is connected directly to power ground; and a large "heatsink" is provided for that transistor.

When conventional current flows from the "run-start" contact of switch82 via conductor 87, stationary and movable contacts 162 and 164 ofswitch 174 in FIG. 6B, conductor 199, field winding 54, conductor 197,the secondary winding of rate transformer 70 in FIG. 6C, conductor 193,movable and stationary contacts 170 and 168 of switch 174 in FIG. 6B,conductor 176 and the collector-emitter circuit of transistor 160 inFIG. 6C to power ground, the polarity at brush 53 will be positive.However, when current flows from that "run-start" contact via conductors87 and 194, stationary and movable relay contacts 188 and 190,stationary and movable contacts 172 and 170 of switch 174, conductor193, the secondary winding of rate transformer 70 in FIG. 6C, conductor197, field winding 54 in FIG. 6B, conductor 199, movable and stationarycontacts 164 and 166 of switch 174, movable and stationary relaycontacts 184 and 182, conductor 176, and the collector-emitter circuitof transistor 160 in FIG. 6C to power ground, the polarity at brush 53will be negative. During "start up" of the engine 56, the logic "1" atthe output of Norton amplifier 136 in FIG. 6B will cause Nortonamplifier 152 in FIG. 6C to apply a logic "1" to the base of transistor156; and that transistor and transistor 160 will be conductive at thesaturation level. As many as four amperes can flow through field winding54 in FIG. 6B during starting of the engine 56; and such a flow ofcurrent will enable that winding to develop a magnetic field which willinteract with the magnetic field that is generated by armature 52 toeffect rapid rotation of that armature. The resulting rotation of thecrankshaft engine 56 will initiate starting of that engine.

The movable contacts 164 and 170 of polarity-reversing switch 174 inFIG. 6B are shown in "reverse" positions which enable the flow ofcurrent through field winding 54 to develop a positive voltage at brush53, and hence at output terminals 64 and 66 in FIG. 6C. When thosemovable contacts are in their "straight" positions, wherein they engagestationary contacts 166 and 172 in FIG. 6B, they will tend to cause theflow of current through field winding 54 to provide a negative polarityat brush 53, and hence at the output terminals 64 and 66. Although anegative polarity at those terminals is required for some weldingoperations, such a polarity would be unacceptable during starting of theengine 56. The relay coil 180 in FIG. 6B permits the polarity of thevoltage at brush 53 to be negative during some welding operations butwill always keep that polarity positive during starting of the engine56. Specifically, as long as the output of Norton amplifier 136 is alogic "1", as it will be until the engine crankshaft reaches apredetermined speed, that logic "1" will cause transistors 140 and 86 toconduct at the saturation level; and hence current will flow from the"run-start" contact of switch 82 via conductors 87 and 194, relay coil180, junction 84 and the collector-emitter circuit of transistor 86 toground to energize that relay coil. The resulting positioning of movablerelay contacts 184 and 190 in engagement with relay contacts 186 and192, and out of engagement with relay contacts 182 and 188, will permitconventional current flow only from field winding 54 to the secondarywinding of rate transformer 70 and not vice versa, even if the operatorof the welder leaves, or places, the movable contacts 164 and 170 intheir "straight" positions. Specifically, current will flow from the"run-start" contact of switch 82 via conductors 87 and 194, stationaryand movable relay contacts 186 and 184, stationary and movable contacts166 and 164, conductor 199, field winding 54, conductor 197, thesecondary winding of rate transformer 70 in FIG. 6C, conductor 193,movable and stationary contacts 170 and 172, movable and stationaryrelay contacts 190 and 192, conductor 176, and the collector-emittercircuit of transistor 160 in FIG. 6C to power ground. That current flowhas the same direction, and causes the field winding 54 to produce thesame polarity at brush 53, and hence at terminals 64 and 66, as thecurrent flow which occurs when the movable contacts 164 and 170 are intheir "reverse" positions. It thus should be apparent that regardless ofthe position in which the operator sets, or leaves, thepolarity-reversing switch 174 during starting of the engine 56, thepolarity at the brush 53 will be positive; and hence the generator 50can act as a motor to start that engine. It would be undesirable to haverelay coil 180 energized at any time other than during the time whenengine 56 is being started. Consequently, that coil will becomede-energized after the engine has reached a predetermined minimum speed,because the output of Norton amplifier 136 will then become a logic "0"and permit transistor 86 to become non-conductive and therebyde-energize relay coil 180.

A diode 195 in FIG. 6B has the anode thereof connected to conductor 194and has the cathode thereof connected to conductor 79. As a result, thatdiode is connected between junction 81 and the "run-start" contact ofswitch 82. This is desirable; because it will enable that diode todissipate any inductive surges which might develop between that junctionand that contact as that switch was operated.

A diode 165 in FIG. 6C has the anode thereof connected to the movablecontact 164 of switch 174 in FIG. 6B by conductor 199, and has thecathode thereof connected to conductor 79; and a diode 167 in FIG. 6Chas the anode thereof connected to power ground, and has the cathodethereof connected to movable contact 164 by conductor 199. If thevoltage at movable contact 164 tends to exceed the voltage on conductor79 by more than about one-half to three-quarters of a volt, diode 165will become conductive and will clamp the voltage at that movablecontact to substantially the value of the voltage on conductor 79. Onthe other hand, if the voltage at movable contact 164 tends to dropbelow the value of power ground by more than about one-half tothree-quarters of a volt, diode 167 will become conductive and willclamp the voltage at that movable contact to substantially the value ofpower ground. Consequently, diodes 165 and 167 will keep the voltage atmovable contact 164 from rising appreciably above the value of thevoltage on conductor 79 and from falling appreciably below the value ofpower ground.

A diode 169 in FIG. 6C has the anode thereof connected to the movablecontact 170 of switch 174 in FIG. 6B by conductor 193, and has thecathode thereof connected to conductor 79; and a diode 171 has the anodethereof connected to power ground and has the cathode thereof connectedto movable contact 170 by conductor 193. If the voltage at movablecontact 170 tends to exceed the voltage on conductor 79 by more thanabout one-half to three-quarters of a volt, diode 169 will becomeconductive and will clamp the voltage at that movable contact tosubstantially the value of the voltage on conductor 79. On the otherhand, if the voltage at movable contact 170 tends to drop below thevalue of power ground, diode 171 will become conductive and will clampthe voltage at that movable contact to substantially the value of powerground. Consequently, diodes 169 and 171 will keep the voltage atmovable contact 170 from rising appreciably above the value of thevoltage on conductor 79 and from falling appreciably below the value ofpower ground.

WELDING LEVEL CONTROL CIRCUIT

The amount of current which will be permitted to flow through the fieldwinding 54 in FIG. 6B, and hence the amount of power which is developedby the generator 50, is a function of the conductivity of transistor 160in FIG. 6C. The conductivity of that transistor is, in turn, a functionof the conductivity of transistor 156. As pointed out hereinbefore, theoutput of Norton amplifier 152 in FIG. 6C controls the conductivity oftransistor 156; and, during the starting of engine 56, that output isdetermined by the Solenoid Disconnecting Circuit. After that engine hasbeen started, other sub-circuits will control the signal which isapplied to the base of transistor 156; and hence those sub-circuits alsowill control the conductivity of transistor 160.

One of those sub-circuits includes Norton amplifiers 196 and 198 in FIG.6C, a capacitor 200, and resistors 202, 204, 206, 208 and 210. Thatsub-circuit operates as a "chopper" oscillator; and it provides agenerally-triangular wave form which is supplied to the non-invertinginput of Norton amplifier 152 to cause that Norton amplifier to supplypulses to the base of transistor 156. The time intervals between thosepulses are desirable, because they limit the duty cycle of transistor156 and, importantly, limit the duty cycle of transistor 160. Thelimitation on the duty cycle of the latter transistor is great enough toenable the large heat sink which is associated with that transistor tohold the temperature of that transistor down to an acceptable level.Although the output of the chopper oscillator sub-circuit is agenerally-triangular wave form, Norton amplifier 152 will provide pulsesthat have steep sides and that have widths which vary in accordance withany signals that are applied to the inverting input of that Nortonamplifier. Although Norton amplifier 152 can respond to wave forms ofdifferent frequencies, it has been found that a generally-triangularwave form with a frequency of one thousand Hertz is very useful. Furthersub-circuits which help determine the output of Norton amplifier 152 arethe Hot Start Circuit, the Direct-Reading Command Circuit, the CurrentFeedback Circuit, the Voltage Feedback Circuit, and the Current-VoltageFeedback Circuit. Those sub-circuits are described hereinafter inappropriately-entitled sections.

HOT START CIRCUIT

A Norton amplifier 212 in FIG. 6C has the output thereof connected tothe inverting input of Norton amplifier 152 by a diode 214 and aresistor 216. The inverting input of Norton amplifier 212 is connectedto junction 105 by conductor 107, an adjustable resistor 218 and aresistor 220. An integrating capacitor 222 and a resistor 224 areconnected between the output and input of Norton amplifier 212. Aresistor 226 connects the non-inverting input of Norton amplifier 212 toa conductor 227 which extends to the Voltage Feedback Circuit in FIG.6A. Pin 7 of Norton amplifier 212 is connected to signal ground; and pin14 of that amplifier is connected to junction 105 by conductor 107.

DIRECT-READING COMMAND CIRCUIT

The numeral 234 in FIG. 6B denotes a regulator which is part of athree-digit, three thumbwheel, BCD switch 233; and the input of thatregulator is connected to junction 105 by conductor 107. The "ground"terminal of that regulator is connected to signal ground, and also tothe lower terminal of shunt 72, by a conductor 236. The output of thatregulator is connected to one of two spaced terminals 242, a resistor238, and to movable contacts 252, 254, 256, 258, 268, 270, 272, 274,284, 286, 288 and 290 of that thumbwheel switch. The other terminal 242is connected to the lower terminal of resistor 238 and to the base of aPNP transistor 246. Resistors 260, 262, 264, 266, 276, 278, 280, 282,292, 294, 296 and 298, respectively, connect the collector of transistor246 to stationary contacts 252, 254, 256, 258, 268, 270, 272, 274, 284,286, 288 and 290 of that thumbwheel switch. A resistor 240 is connectedbetween the base of transistor 246 and conductor 236; and spacedterminals 244 are connected in parallel with that resistor. A resistor248 is connected between the collector of transistor 246 and conductor236; and spaced terminals 250 are connected in parallel with thatresistor. Trimming resistors can be connected across spaced terminals242, 244 and 250 to compensate for variations in the voltage-settingcapability of regulator 234 and in the resistive values of resistors238, 240 and 248, respectively.

Resistors 260, 262, 264 and 266 are associated with the "hundreds"thumbwheel, resistors 276, 278, 280 and 282 are associated with the"tens" thumbwheel, and resistors 292, 294, 296 and 298 are associatedwith the "units" thumbwheel. When only contact 290 of the thumbwheelswitch 233 is closed, the current flowing through resistor 298 and theemitter-collector circuit of transistor 246 will cause the welder totend to supply one ampere of welding current to the electrode andworkpiece when that welder is in its constant current mode and to supplyone-tenth of a volt to that electrode and workpiece when that welder isin its constant voltage mode. Similarly, when only contact 288, 286,284, 274, 272, 270, 268, 258, 256, 254 or 252 is closed, the currentflowing through resistor 296, 294, 292, 282, 280, 278, 276, 266, 264,262 or 260, respectively, will tend to be two, four, eight, ten, twenty,forty, eighty, one hundred, two hundred, four hundred and eight hundredamperes when that welder is in its constant current mode and to supplytwo-tenths, four-tenths, eight-tenths, one, two, four, eight, ten,twenty, forty and eighty volts to that electrode and workpiece when thatwelder is in its constant voltage mode. Proper setting of the threethumbwheels of switch 233 can establish a command for any desired valueof current between one ampere and nine hundred and ninety-nine amperes.

It will be noted that the value of resistor 264 is more than a fulldecade below the value of resistor 280, it will be noted that the valueof resistor 262 is more than a full decade below the value of resistor278, and it will be noted that the value of resistor 260 is more than afull decade below the value of resistor 276. Further, it will be notedthat the value of resistor 160 is less than, rather than being exactly,one-half of the value of resistor 262; and the value of resistor 264 isless than, rather than being exactly, one-half of the value of resistor266. The less than one-half values of resistors 260, 262 and 264compensate for the increased voltage drop which develops across theemitter-base circuit of transistor 246 at welding current levels inexcess of close to two hundred amperes.

The voltage at the base of transistor 246 is fixed by resistors 238 and240 and by any trimming resistors that are connected across the spacedterminals 242 and 244. The collector load will be fixed because it willconsist of resistor 248 plus any trimming resistor that may be connectedacross the spaced terminals 250. However, the emitter load can be anyone of a large number of predetermined values as selected by thesettings of the thumbwheels of thumbwheel switch 233; and transistor 246will respond to those various values of emitter load to be conductive atdifferent levels of conductivity. The welder will respond to thosedifferent levels of conductivity to tend to supply welding currentlevels which exactly equal the visual digital reading provided by thatthumbwheel switch when that welder is in its constant current mode andto supply voltages which are exactly one-tenth of that visual digitalreading when that welder is in its constant voltage mode. In this way,the present invention is able to use inexpensive and compact componentsto set, and to visually check, precise levels of welding current and ofwelding voltage to be supplied by the welder.

MODE SELECTING CIRCUIT

The direct-reading command signal from the Direct-Reading CommandCircuit is always modified by one or more feedback signals. Whenever afeedback signal is solely responsive to the value of the weldingcurrent, it is obtained from the shunt 72, whenever a feedback signal issolely responsive to the value of the welding voltage, it is obtained bysensing the voltage across the output terminals 66 and 68, and whenevera feedback signal is responsive to the value of the welding current andto the value of the welding voltage, it is obtained from theCurrent-Voltage Feedback Circuit which senses the outputs of the CurrentFeedback and Voltage Feedback Circuits.

The numeral 312 in FIG. 6A denotes a mode selecting switch that hasthree stationary contacts; and the upper of those contacts is connectedto the Current Feedback Circuit, the lower of those contacts isconnected to the Voltage Feedback Circuit, and the middle of thosecontacts is connected to the Current-Voltage Feedback Circuit which canprovide a signal that is responsive to both the Current Feedback andVoltage Feedback Circuits. Conductor 313 in FIGS. 6A and 6B andresistors 314 and 318 in FIG. 6B connect the movable contact of switch312 of FIG. 6A to the non-inverting input of an operational amplifier302 in FIG. 6A. A capacitor 316 is connected between signal ground andthe junction of those resistors; and that capacitor and resistor 314will bypass high frequency transients to signal ground.

A conductor 300 and a resistor 304 in FIG. 6B connect the collector oftransistor 246 of the Direct-Reading Command Circuit to the invertinginput of amplifier 302. A capacitor 306 is connected between conductor300 and signal ground to by-pass high frequency transients to signalground. An integrating capacitor 310 and a resistor 308 are connectedbetween the input and output of amplifier 302. That amplifier willrespond to the signals applied to the inverting and non-inverting inputsthereof to provide an output which is a composite of the command signalfrom the Direct-Reading Command Circuit and of the signal from themode-selecting switch 312. That output will be applied to a conductor303 which extends from FIG. 6B into FIG. 6C.

CURRENT FEEDBACK CIRCUIT

Referring particularly to FIG. 6A, the numeral 325 generally denotes aCurrent Feedback Circuit. A resistor 328 and a capacitor 326 of thatcircuit are connected in parallel between signal ground and powerground, and are connected to the lower terminal of shunt 72 by conductor236. That capacitor and resistor will tend to keep signal ground andpower ground at the same potential despite high frequency and othertransients.

The numerals 337, 340 and 356 denote operational amplifiers which areparts of the Current Feedback Circuit 325. The "grounding" pins of thoseamplifiers are connected to signal ground; and pins 4 of theseamplifiers are connected to junction 105 by conductor 107. A resistor330 and a capacitor 338 bypass to signal ground any high frequencytransients which appear across the shunt 72 and which are applied toresistor 330 by a conductor 324. That conductor, that resistor, and aresistor 332 apply each signal at the upper end of shunt 72 in FIG. 6Bto the inverting input of operational amplifier 340 in FIG. 6A. Aresistor 342 connects the non-inverting input of that operationalamplifier to signal ground; and a diode 344 and a resistor 334 connectthe output of amplifier 340 to the inverting input of that amplifier. Itwill be noted that resistors 332 and 334 have the same values and areone percent resistors. This means that if the input to resistor 330 isnegative, the signal at the cathode of diode 344 will be a positivevalue which is equal in magnitude to the signal at the input of resistor330. Also, it means that if the input to resistor 330 is positive, thesignal at the cathode of diode 344 will again be a positive value;because amplifier 340 will back-bias that diode and thereby permitresistors 332 and 334 to apply the positive signal from conductor 324 tothe cathode of that diode. A resistor 336 applies signals at the cathodeof diode 344 to the non-inverting input of amplifier 337.

To enable the value of the welding current supplied by output terminals66 and 68 to match the values of current called for by the thumbwheelswitch 233, the electric welder must develop precise current feedbacksignals. To make it possible to develop such signals, either the shunt72 must have a precise predetermined value of resistance or the CurrentFeedback Circuit must be capable of being adjusted to be usable with ashunt which does not have that precise predetermined value ofresistance. Because a large capacity shunt which has a precisepredetermined value of resistance is very expensive, the CurrentFeedback Circuit of the present invention is made so it can developprecise current feedback signals even though it is used with a shuntthat does not have a precise predetermined value of resistance.

An adjustable resistor 346 and a resistor 350 in FIG. 6A are connectedbetween the output of amplifier 337 and signal ground to act as avoltage-dividing feedback loop for that amplifier; and a resistor 348connects the junction between adjustable resistor 346 and resistor 350to the inverting input of amplifier 337. Further the adjustable resistor346 is selected so the maximum resistance thereof is about one-half ofthe resistance of resistor 350. The output of amplifier 337 is connectedto the non-inverting input of amplifier 356, a resistor 352 is connectedbetween that output and signal ground, and a resistor 358 is connectedbetween the output of the latter amplifier and signal ground. Aconductor connects the output of amplifier 356 to the inverting input ofthat amplifier to enable that amplifier to operate as a unity gainbuffer.

To enable the value of the welding current supplied by output terminals66 and 68 to match the values of current called for by the thumbwheelswitch 233, when the welder is in its constant current mode, the CurrentFeedback Circuit should respond to a predetermined flow of currentthrough shunt 72 to provide a predetermined voltage at the output ofamplifier 356--even if the resistance of that shunt does not preciselymeet a predetermined value. In the preferred embodiment of the presentinvention, the Current Feedback Circuit should respond to a flow of oneampere through shunt 72 to provide a voltage of five millivolts at theoutput of amplifier 356--even if the resistance of that shunt variesfrom a high of five thousandths (0.005) of an ohm to a low ofthirty-four thousand, three hundred and seventy-five ten-millionths(0.003475) of an ohm. By making the maximum resistance of adjustableresistor 346 about one-half that of resistor 350--and in the saidpreferred embodiment, the maximum resistance of adjustable resistor 346is one hundred kilohms and the resistance of resistor 350 is two hundredand twenty kilohms, the present invention enables that adjustableresistor and that resistor to vary the gain of amplifier 337 from unityto about one and one-half. If the resistance of a moderately-priced,commercially-available resistor which is to be used as the shunt 72 isfive thousandths (0.005) of an ohm, the movable contact of adjustableresistor 346 will be set in its extreme right-hand position to provideunity gain for amplifier 337; and the Current Feedback Circuit willdevelop five millivolts at the output of amplifier 356 for each ampereof current flowing through shunt 72. If the resistance of amoderately-priced, commercially-available resistor which is to be usedas the shunt 72 is thirty-four thousand, three hundred and seventy-fiveten-millionths (0.0034375) of an ohm, the movable contact of adjustableresistor 346 will be set so the gain of amplifier 337 will be slightlygreater than one and forty-five hundredths (1.45). If the resistance ofa moderately-priced, commercially-available resistor which is to be usedas the shunt 72 is greater than thirty-four thousand, three hundred andseventy-five ten-millionths (0.0034375) of an ohm but is less than fivethousandths (0.005) of an ohm, the movable contact of adjustableresistor 346 will be set at a point between the settings correspondingto shunts having those values to provide five millivolts at the outputof amplifier 356 when one ampere of current is flowing through shunt 72.

To set the movable contact of adjustable resistor 346, the thumbwheelswitch 233 will be adjusted, regardless of the current being displayedthereby, until the ammeter 75 shows that one ampere of current isflowing through shunt 72. Thereupon, that movable contact will beshifted to the position wherein the voltage at the output of amplifier356 is five millivolts.

The amplifier 356 applies its output to conductor 360 which extends tothe upper contact of mode-selecting switch 312. When the movable contactis in engagement with that upper contact, the command signal from theDirect-Reading Command Circuit will be modified by a current feedbacksignal. All of this means that the Current Feedback Circuit of thepresent invention enables the value of welding current supplied byoutput terminals 66 and 68 to match the values of current called for bythe thumbwheel switch 233 even though the resistance of the shunt mayvary from a high of five thousandths (0.005) of an ohm to a low ofthirty-four thousand, three hundred and seventy-five ten-millionths(0.0034375) of an ohm. As a result, the electric welder 10 can use amoderately-priced, commercially-available resistor as the shunt 72 andstill provide precise current feedback.

VOLTAGE FEEDBACK CIRCUIT

The Voltage Feedback Circuit is generally denoted by the numeral 359 inFIG. 6A. That Circuit includes an operational amplifier 370 and anoperational amplifier 376. A low value resistor 362 and a capacitor 372are connected as a series filter to bypass to signal ground anyhigh-frequency transients in the signal which is supplied to thatresistor by a conductor 361 which extends from the output terminal 66 inFIG. 6C. Resistors 364 and 374 in FIG. 6A form a voltage divider for thesignal which is developed at the upper terminal of capacitor 372; andthe junction of those resistors is connected to the inverting input ofamplifier 376. A diode 380 and a resistor 366 connect the output of thatamplifier to the inverting input of that amplifier. The resistance ofresistor 366 in one-twentieth that of resistor 364; and those resistorsand resistor 374 are one percent resistors. A resistor 378 connects thenon-inverting input of amplifier 376 to signal ground.

A resistor 368 connects the cathode of diode 380 to the non-invertinginput of amplifier 370. A resistor 382 connects the output of thatamplifier to the inverting input of that amplifier. A resistor 384 isconnected between signal ground and the output of amplifier 370; and theconductor 227 also is connected to that output.

When the voltage at output terminal 66 in FIG. 6C is positive, conductor361 and resistors 362, 364 and 366 in FIG. 6A will apply a positivevoltage to the inverting input of amplifier 376. The resulting negativeoutput of the amplifier will back-bias diode 380, and will therebyrender the feedback loop of that amplifier inactive. The very smallvalue of resistor 362 will enable substantially all of the voltageapplied to that resistor to be dropped across resistors 364 and 374.Because the resistance of resistor 364 is two hundred kilohms whereasthe resistance of resistor 374 is only ten and one-half kilohms, thevoltage drop across resistor 364 will be only one-twentieth of the sumof the voltage drops across resistors 364 and 374. Resistors 366 and 378will couple the voltage at the upper terminal of resistor 374 to thenon-inverting input of amplifier 370. On the other hand, when thevoltage at output terminal 64 in FIG. 6C is negative, conductor 361 andresistors 362, 364 and 374 in FIG. 6A will apply a negative voltage tothe inverting input of amplifier 376. The resulting positive output ofthat amplifier will forward-bias diode 380, and will thereby cause thatamplifier to tend to hold the inverting input thereof at virtual signalground. This means that resistor 374 will have essentially the samevoltages at the upper and lower terminals thereof, and hence can notprovide a scaled-down voltage for the non-inverting input of amplifier370. Instead, feedback resistor 366 will respond to the flow of feedbackcurrent therethrough to develop a positive voltage at the cathode ofdiode 380; and resistor 368 will apply that voltage to the non-invertinginput of amplifier 370. Because the resistance of resistor 366 isone-twentieth of the resistance of resistor 364, the voltage which willbe developed at the cathode of diode 380, and then will be supplied tothat non-inverting input by resistor 368, will be one-twentieth of thevoltage which conductor 361 supplies to resistor 362.

Amplifier 370 acts as a unity gain buffer; and hence that amplifier willcause conductor 386 to apply to the lower contact of mode-selectingswitch 312 the same voltage that is applied to the non-inverting inputof that amplifier. As indicated hereinbefore, the voltage which isapplied to that non-inverting input will--whether the voltage whichconductor 361 senses at output terminal 64 in FIG. 6C and applies toresistor 362 is positive or negative--be a positive voltage that isone-twentieth of the voltage at that output terminal. This means thatwhen the movable contact of the mode-selecting switch is set inengagement with the lower contact of that switch, the command signalfrom the Direct-Reading Command Circuit will be modified by a positivevoltage feedback signal which is one-twentieth of the value of thevoltage across output terminals 66 and 68.

CURRENT-VOLTAGE FEEDBACK CIRCUIT

Some welding operations are best performed when the feedback signal forthe welder includes both a voltage component and a current component;and the Current-Voltage Feedback Circuit of the present invention canprovide a feedback signal which includes both of those components. Thatcircuit is denoted by the numeral 387 in FIG. 6A; and it includes apotentiometer 319 which has one terminal thereof connected to the outputof amplifier 356 of Current Feedback Circuit 325 by conductor 360 andwhich has the other terminal thereof connected to the output ofamplifier 370 of Voltage Feedback Circuit 358 by conductor 386. Themovable contact of that potentiometer is connected to the non-invertinginput of an operational amplifier 390 by a resistor 398. Resistors 392and 394 constitute a voltage divider which is connected between theregulated eight volts and signal ground by conductor 113. The junctionbetween those resistors is connected to the inverting input of amplifier390; and a resistor 396 connects the output of that amplifier to thatinput of that amplifier.

Resistor 396 has a value of eighty-two kilohms, resistor 392 has a valueof six hundred and eighty kilohms, resistor 394 has a value of onehundred kilohms, and potentiometer 319 has a value of one hundredkilohms. The voltage drops across resistors 392 and 394 provide a onevolt offset at the inverting input of amplifier 390. Also, thoseresistors simulate an input of eighty-two kilohms for that invertingamplifier; and, because resistor 396 has a value of eighty-two kilohms,amplifier 390 has a gain of two.

When the contact of potentiometer 319 is set in its middle position, anycurrent feedback signal which is applied to the upper end thereof byconductor 360 will be attenuated to one-half of its value, and anyvoltage feedback signal which is applied to the lower end thereof byconductor 386 also will be attenuated to one-half of its value. Theresulting signal which is applied to the non-inverting input ofamplifier 390 will be doubled in value by that amplifier; and hence thesignal at the output of amplifier 390 will have in it the same currentfeedback component which conductor 360 applied to potentiometer 319, andalso will have in it the same voltage feedback component which conductor386 applied to that potentiometer. It should be noted that because theVoltage Feedback Circuit provides a one to twenty scaling down of thevoltage applied to resistor 362 by conductor 361, before that voltage isapplied to conductor 386, the one volt offset at the inverting input ofamplifier 370 corresponds to a twenty volt welding voltage at outputterminal 66 in FIG. 6C.

The amplifier 390 will respond to various settings of the movablecontact of potentiometer 319 to provide varying ratios of currentfeedback signals and voltage feedback signals and hence will providevarying "slopes" on a graph wherein welding voltage is plotted againstwelding current.

However, when that movable contact is in its center position, amplifier390 and that movable contact will provide a "slope" of essentiallyforty-five degrees on a linear plot when a vertical scale of one volthas the same length as a horizontal scale of ten amperes. Also, whenthat movable contact is in its center position, the welder will, whenthe welding voltage is twenty volts, provide welding current valueswhich precisely match the settings of the thumbwheel switch 233.

A resistor 400 and a conductor 404 connect the output of amplifier 390to the center contact of mode-selecting switch 312. A capacitor 402bypasses to signal ground any high frequency transients which passthrough resistor 400. All of this means that when the movable contact ofmode-selecting switch 312 is connected to the center contact of thatswitch, the command signal from the Direct-Reading Command Circuit willbe modified by a combined current feedback and voltage feedback signal.

LOAD-SENSING CIRCUIT

Referring particularly to FIGS. 6A and 6B, the numeral 482 denotes asolenoid which selectively permits the throttle of engine 56 to be inits "idle" position or in a preset "full output" position. Althoughdifferent solenoids could be used, one particularly useful solenoid is astep stage solenoid which has an internal switch 484. One terminal ofthat solenoid is connected to conductor 79; and the other terminal ofthat solenoid is connected to a junction 485. NPN transistors 474 and476 constitute a Darlington amplifier which has its output connected tojunction 485. A diode 478 and a capacitor 480 are connected in seriesbetween the upper terminal of solenoid 482 and power ground; and theywill act as an energy-dissipating circuit. The junction between diode478 and capacitor 480 is connected to junction 81 by conductor 79. Theemitter of transistor 476 is connected to power ground.

The numeral 466 denotes a PNP transistor which has the emitter thereofconnected to junction 105 by conductor 107, and which has the collectorthereof connected to power ground by series-connected resistors 468, 470and 472. Resistors 462 and 464 are connected in series between conductor107 and a "jumper" 460 in FIG. 6A; and those resistors are selectivelyconnected to a "full output" terminal 456 or to an "idle" terminal 458in FIG. 6A by a conductor 488 and that "jumper".

The numerals 408 and 418 in FIG. 6A denote a resistor and capacitorwhich constitute a series filter that is connected between the output ofoperational amplifier 337 of the Current Feedback Circuit 325 and signalground by a conductor 354. An operational amplifier 406, which is usedas a comparator, has the non-inverting input thereof connected to theupper terminal of capacitor 418 by a resistor 410. A resistor 416connects the inverting input of that operational amplifier to themidpoint of a voltage divider which includes resistors 412 and 414 andwhich is connected between the source of regulated eight volts andsignal ground by conductor 113. The output of that operational amplifieris connected to one input of a NAND gate 430 by resistors 420 and 422and junction 423. A manually-operated switch 424 and a resistor 426 canselectively connect junction 423 to junction 105 by conductor 107. Acapacitor 428 is connected between junction 423 and power ground; andthat capacitor and resistor 426 will bypass to ground any high frequencytransients which might reach that resistor. When switch 424 is open, thevoltage at junction 423 will largely be a function of the output ofoperational amplifier 406. The "grounding" terminal of operationalamplifier 406 is connected to signal ground; and the positive voltageterminal of that operational amplifier is connected to junction 105 byconductor 107.

The output of NAND gate 430 is connected to the upper input of a NANDgate 434; and the output of the latter NAND gate is connected directlyto "full output" terminal 456, and also is connected by a resistor 454to the upper input of NAND gate 436 and to the anode of a diode 452. Theoutput of NAND gate 436 is connected to "idle" terminal 458 and to thelower input of NAND gate 434. A resistor 448 and a capacitor 449 areconnected between junction 105 and signal ground by conductor 107; andthe cathode of diode 452 is connected to the upper terminal of thatcapacitor. The transistor 228 has the collector-emitter circuit thereofconnected across that capacitor; and a resistor 232 is connected betweenthe base and emitter of that transistor. An adjustable resistor 438, aresistor 440, and parallel-connected capacitors 444 and 446 areconnected between junction 105 and power ground by conductor 107. Acapacitor 442 is connected between power ground and the junction betweenadjustable resistor 438 and resistor 440. A NAND gate 432 has the inputsthereof connected together and to the upper terminals of capacitors 444and 446 and, by a diode 450, to the output of NAND gate 430. Pins 14 ofNAND gates 430, 432, 434 and 436 are connected to junction 105 byconductor 107; and pins 7 of those NAND gates are connected to powerground.

Prior to the starting of engine 56, the base of transistor 228 in FIG.6A will receive a logic "1" from Norton amplifier 136 in FIG. 6B viaconductor 147 and resistor 230; and the resulting conductivity of thattransistor will keep capacitor 449 discharged. The consequent logic"0's" at the lower input of NAND gate 430 and at the cathode of diode452 will cause NAND gate 430 to develop a logic "1" at the outputthereof and to apply it to the upper input of NAND gate 434, and willforward bias that diode to cause NAND gate 436 to "see" a logic "0" atits upper input and to respond to that logic "0" to develop a logic "1"at the output thereof and to apply it to the lower input of NAND gate434 and to the "idle" terminal 458. NAND gate 434 will respond to thelogic "1's" at its inputs to apply a logic "0" to the "full output"terminal 456. All of this means that before the engine 56 is started,logic "1" will appear at "idle" terminal 458 and logic " 0" will appearat "full output" terminal 456.

Whenever the "jumper" 460 of FIG. 6A is set in engatement with the"idle" terminal 458, the Load-Sensing Circuit will cause the engine 56to start running at its "idling" speed rather than at its "full output"speed as that engine is being started. Once that engine has beenstarted, that Load-Sensing Circuit will continue to apply a signal tothat "jumper" which will tend to cause that engine to operate at its"idling" speed rather than at its "full output" speed until the operatorinitiates a welding operation by causing the electrode to touch theworkpiece. As that welding operation is initiated, the Load-SensingCircuit will receive a signal from the Current Feedback Circuit whichwill automatically cause that Load-Sensing Circuit to apply a signal to"jumper" 460 which will cause engine 56 to operate at its "full output"speed; and it will keep that engine operating at that speed as long asthat welding operation is continued. Moreover if, during that weldingoperation, the operator momentarily moves the electrode away from theworkpiece, that Load-Sensing Circuit will, for a short length of time,continue to apply the signal to "jumper" 460 which will tend to keep theengine operating at its "full output" speed.

Specifically, as long as engine 56 is being started, the SolenoidDisconnecting Circuit will keep transistor 228 in FIG. 6A conductive;and the resulting logic "0's" at the lower input of NAND gate 430 and atthe cathode of diode 452 will cause the Load-Sensing Circuit to apply alogic "1" to "jumper" 460--all as explained hereinbefore. Conductor 488and resistor 464 will apply that logic "1" to the base of transistor 466to keep that transistor non-conductive; and the resulting low voltagedrops across resistors 470 and 472 will be unable to forward-bias thetransistors 474 and 476 of the Darlington amplifier. Consequently,solenoid 482 will remain non-conductive, and will thereby permit engine56 to start operating at its "idling" speed.

After that engine has begun to operate at its "idling" speed, theSolenoid Disconnecting Circuit will cause the output of Norton amplifier136 to change from a logic "1" to a logic "0". Thereupon, transistor 228will become nonconductive; and capacitor 449 can become charged anddevelop a logic "1" at its upper terminal. However, if a weldingoperation has not been initiated, the Current Feedback Circuit 325 willapply a logic "0" to the non-inverting input of operational amplifier406 via conductor 354 and resistors 408 and 410; and that operationalamplifier will respond to that logic "0" to apply a logic "0" to theupper input of NAND gate 430. As a result, even though the SolenoidDisconnecting Circuit changed the logic "1", which it had been supplyingto transistor 228, to a logic "0", the Load-Sensing Circuit willcontinue to keep solenoid 482 de-energized--thereby causing engine 56 tocontinue to operate at its "idling" speed.

When the operator initiates a welding operation by causing the electrodeto contact the workpiece, the Current Feedback Circuit will apply alogic "1" to the non-inverting input of operational amplifier 406 viaconductor 354 and resistors 408 and 410. That operational amplifier willrespond to that logic "1" to cause resistors 420 and 422 to change thelogic "0" at the upper input of NAND gate 430 to a logic "1"; and thatlogic "1" plus the logic "1" which capacitor 449 applies to the lowerinput of that NAND gate will cause that NAND gate to change the logic"1" at its output to a logic "0". NAND gate 434 will respond to theresulting "0" at its upper input to apply a logic "1" to the upper inputof NAND gate 436. The logic "0" at the output of NAND gate 430 willforward bias diode 450; and the resulting logic "0" at the inputs ofNAND gate 432 will cause the latter NAND gate to apply a logic "1" tothe lower input of NAND gate 436. Thereupon, the logic "1" at the"jumper" 460, and hence at the base of transistor 466 in FIG. 6B, willbecome a logic "0". The resulting conductivity of that transistor willenergize solenoid 482, and will thereby cause engine 56 to operate atits "full output" speed. It thus should be apparent that once the engine56 has begun to "run", the Load-Sensing Circuit will cause that engineto "run" at its "idling" speed until a welding operation is initiated,and will automatically cause that engine to "run" at its "full output"speed during a welding operation. Once the welding operation isconcluded, the Current Feedback Circuit will again apply a logic "0" tothe non-inverting input of operational amplifier 406 of the Load-SensingCircuit; and the latter circuit will again cause engine 56 to run at its"idling" speed.

Throughout the duration of the welding operation, the diode 450 of theLoad-Sensing Circuit will be forward-biased, and the voltage at theupper terminals of capacitors 444 and 446 of that circuit will be alogic "0". If the operator momentarily moves the electrode away from theworkpiece, the output of the operational amplifier 337 in the CurrentFeedback Circuit 325 in FIG. 6A will decrease to zero volts; and theoperational amplifier 406 in the Load-Sensing Circuit will respond tothe resulting zero value at the non-inverting input thereof to apply alogic "0" to the upper input of NAND gate 430. Thereupon, the output ofthat NAND gate will become a logic "1", and it will back-bias the diode450. The non-conductive state of that diode will tend to permit thevoltage at the inputs of NAND gate 432 to rise; but adjustable resistor438 and capacitors 442, 444 and 446 will constitute a R.C. network whichwill keep the voltage at those inputs from immediately rising to a valuewhich wll correspond to a logic "1". By appropriately adjusting theposition of the movable contact of that adjustable resistor, it ispossible to keep logic "0" at the inputs of NAND gate 432 for from fourto twenty seconds after the electrode has been moved away from theworkpiece. By delaying the time when that NAND gate will change itsoutput from a logic "0" to a logic "1", the Load-Sensing Circuit enablesNAND gate 436 to continue to apply a logic "0" to the base of transistor466, and thereby enables that transistor to keep solenoid 482energized--with consequent operation of engine 56 at its "full output"speed. If the operator resumes the welding operation before the end ofthe pre-set delay of from four seconds to twenty seconds, the resultinglogic "1" at the output of operational amplifier 337 in Current FeedbackCircuit 325 will enable operational amplifier 406 and NAND gate 430 ofthe Load-Sensing Circuit to again develop a logic "0" at the output ofthat NAND gate--and hence at the cathode of diode 450. The renewedforward-biasing of that diode will continue to maintain logic "0" at theinputs of NAND gate 432 and it will discharge the capacitors 444 and446. NAND gate 432 will apply a logic "1" to the lower input of NANDgate 436 to enable the latter NAND gate to continue to apply logic "0"to the base of transistor 416; and the discharged capacitors will insurea full time delay of from four seconds to twenty seconds when theelectrode is next moved away from the workpiece. In this way, theLoad-Sensing Circuit keeps momentary pauses in a welding operation fromcausing engine 56 to drop from its "full output" speed to its "idling"speed.

It should be noted that the resistor 448 and the capacitor 449 areuseful in making certain that the Load-Sensing Circuit automaticallyapplies a logic "1" to "jumper" 460 when the welder 10 is turned "on".Specifically, that capacitor will be in a discharged state prior to thetime the welder is turned "on"; and that capacitor will not chargepromptly as that welder is turned "on". In the first place, the SolenoidDisconnecting Circuit will keep transistor 228 conductive, and willthereby keep that capacitor "shorted", until the engine 56 is started;and, thereafter, the resistor 448 will limit the rate at which thatcapacitor can become charged. Throughout the time that capacitor is notcharged, it will forward bias diode 452 to apply a logic "0" to theupper input of NAND gate 436; and the resulting logic "1" at the outputof that NAND gate will keep transistor 466 in FIG. 6B non-conductive andwill thereby keep solenoid 482 de-energized. All of this means that atturn "on", and until the engine 56 has been started, the Load-SensingCircuit will keep solenoid 482 de-energized.

The NAND gates 430, 432, 434 and 436 act as a latch or flip-flop. Atturn "on", those NAND gates will apply a logic "1" to "jumper" 460, andthey will continue to apply that logic "1" to that "jumper" until theengine 56 has started and a weld has been initiated. Thereafter, thoseNAND gates will apply a logic "0" to that "jumper"; and they willcontinue to apply that logic "0" to that "jumper" until the weldingoperation has been interrupted for a period of time which is longer thanthe four second to twenty second time delay provided by adjustableresistor 438 and capacitors 442, 444 and 446.

In keeping the engine 56 operating at its "full output" speed for fromfour to twenty seconds after the electrode has been moved away from theworkpiece, the Load-Sensing Circuit avoids the delays which would resultif that engine dropped to its "idling" speed as soon as that electrodewas moved away from that workpiece. Also, that Load-Sensing Circuitenables the operator of the welder 10 to make more uniform welds than hecould make if the engine 56 dropped to its "idling" speed, and then hadto increase its speed to the "full output" level, each time theelectrode was moved away from the workpiece.

The switch 424, in the Load-Sensing Circuit of FIG. 6A, can be closed tocause the engine 56 to operate continuously at its "full output" speed.Specifically, when that switch is closed, it will apply a continuouslogic "1" to the upper input of NAND gate 430; and that continuous logic"1" will coact with the continuous logic "1" which capacitor 449 appliesto the lower input of that NAND gate to cause that NAND gate to developa continuous logic "0" at its output. Diode 450 will respond to theresulting continuous logic "0" at its cathode to provide a continuouslogic "0" at the inputs of NAND gate 432. That NAND gate will respond tothose logic "0's", and NAND gate 434 will respond to the logic "0" atthe upper input thereof, to apply continuous logic "1's" to the inputsof NAND gate 436. That NAND gate will then apply a continuous logic " 0"to the base of transistor 466 in FIG. 6B--with consequent energizationof solenoid 482 and with resulting "full output" speed operation ofengine 56.

It should be noted that even where the switch 424 in FIG. 6A is closedto cause the engine 56 to operate continuously at "full output" speed,the resistor 448 and the capacitor 449 will cause the Load-SensingCircuit to maintain a logic "1" at "jumper" 460 for a short period oftime after that engine has started to "run". Specifically, until thatengine is started, the transistor 228 will be kept conductive by theSolenoid Disconnecting Circuit and will keep capacitor 449 "shorted".Even after the engine 56 has been started, that capacitor will forwardbias diode 452--and thereby keep logic "0" at the upper input of NANDgate 436, until that capacitor has become charged. As a result, theLoad-Sensing Circuit of the present invention will keep the engine 56from starting at its "full output" speed, and, instead, will requirethat engine to start at its "idling" speed.

If, as almost always will be the case, the engine 56 is an engine whichtends to "run" at its "idling" speed and must have the throttle thereofmoved to its "full output" position by solenoid 482 in FIG. 6B, the"jumper" 460 will be left in engagement with the "idle" terminal 458.However, in the unusual event the engine 56 is an engine which tends to"run" at its "full output" speed and must have the throttle thereofmoved to its "idle" position by solenoid 482, the "jumper" 460 will beset in engagement with the "full output" terminal 456. In that event,the Load-Sensing Circuit would respond to the rendering of transistor228 thereof conductive, by the Solenoid Disconnecting Circuit, to applylogic "0" to the base of transistor 466 in FIG. 6B via "full output"terminal 456, "jumper" 460, conductor 488, and resistor 464. As a resultduring "start up", that transistor would be conductive and the solenoid482 would be energized, and hence would cause the engine 56 to operateat its "idling" speed. After that engine had started, and theLoad-Sensing Circuit became able to respond to the signal which theCurrent Feedback Circuit 325 supplied via conductor 354, the formercircuit would apply a logic "1" to the base of transistor 466 via "fulloutput" terminal 456, "jumper" 460, conductor 488, and resistor 464. Asa result, that transistor would be non-conductive and the solenoid 482would be deenergized, and hence would cause the engine 56 to operate atits "full output" speed.

All of this means that the Load-Sensing Circuit of the present inventioncan be used with engines which tend to operate at "full output" speed aswell as with engines which tend to operate at "idling" speed. In thisway, that Load-Sensing Circuit makes the welder 10 quite versatile.

OPERATION OF WELDER

The welder provided by the present invention has some sub-circuits whichare always connected across the battery 62; but those sub-circuitseither do not draw any current, or draw so very little current, when theignition switch 82 of FIGS. 3 and 6B is in its "off" position, thatthose sub-circuits do not constitute an unacceptable drain on thatbattery. Those sub-circuits include the alternator 60 of FIGS. 4 and 6B,the collector-emitter circuit of transistor 89 of FIG. 6C, Zener diode91 and the base-emitter circuit of transistor 89 of FIG. 6C, Zener diode91 and resistor 93 of FIG. 6C, diodes 165 and 167 of FIG. 6C, diodes 169and 171 of FIG. 6C, diode 159 and the collector-emitter circuit oftransistor 160 in FIG. 6C, capacitor 480 in FIG. 6B, diode 478 and thecollector-emitter circuit of transistor 476 in FIG. 6B, diode 478 andthe collector-emitter circuit of transistor 474 and the base-emittercircuit of transistor 476 in FIG. 6B, diode 478 and thecollector-emitter circuit of transistor 474 and resistor 472 in FIG. 6B,diodes 90 and 88 in FIG. 6B, diode 90 and the collector-emitter circuitof transistor 86 in FIG. 6B, diode 195 in FIG. 6B and conductor 194 andthe collector-emitter circuit of transistor 95 and then variously viacapacitor 109 in FIG. 6C and via diode 103 and paralleled capacitor 101and Zener diode 99 in FIG. 6C, diode 195 in FIG. 6B and conductor 194and resistor 97 and the base-emitter circuit of transistor 95 and thenvariously via capacitor 109 in FIG. 6C and via diode 103 and paralleledcapacitor 101 and Zener diode 99 in FIG. 6C, and diode 195 in FIG. 6Band conductor 194 and resistor 97 and then through parallel-connectedZener diode 99 and capacitor 101.

Diode 159 limits the voltage which can be applied to the collector oftransistor 160 to less than one volt above the voltage at the positiveterminal of battery 62. As a result, that diode protects that transistoragainst any potentially-hurtful transients which might develop as therelay contacts 184 and 190 moved or as the switch contacts 164 and 170moved.

Before starting the engine 56, the operator should set the choke 26 onthe front panel 12 to a desired setting, set the switch 424 of FIGS. 3and 6A in its "open" position, set the desired value of welding currentor voltage on the thumbwheel switch 233 of FIGS. 3 and 6B, set themode-selecting switch 312 of FIGS. 3 and 6A in the desired position, setthe movable contact of potentiometer 319 of FIGS. 3 and 6A, set switch178 of FIGS. 3 and 6C in the desired position, and set thepolarity-reversing switch 174 of FIGS. 3 and 6B in the desired position,plug the cable for the electrode into terminal 64 or 66 of FIGS. 3 and6C, plug the cable for the workpiece clamp into terminal 68 of FIGS. 3and 6C, insert the key into the ignition switch 82 of FIGS. 3 and 6B,and turn that ignition switch to its "start" position. As the movablecontact 85 of that switch reaches the "run-start" contact, the fuse 80,the junction 81, those contacts, and conductor 87 will connect thepositive terminal of battery 62 to contact 162 and the conductor 194 inFIG. 6B and to resistor 158 in FIG. 6C. The resulting flow of currentfrom that "run-start" contact, via conductors 87 and 194, relay coil180, junction 84, and the collector-emitter circuit of transistor 86will energize that relay coil and shift relay contacts 184 and 190 outof engagement with relay contacts 182 and 188, respectively--therebypreventing any flow of current from the secondary winding of ratetransformer 70 to field winding 54--even if the operator left the switch174 in its "straight" position. Also, current will flow from the"run-start" contact via conductor 87 either through contacts 162 and164, conductor 199, field winding 54, conductor 197, the secondarywinding of rate transformer 70, conductor 193, contacts 170 and 168,conductor 176, and the collector-emitter circuit of transistor 160 topower ground or through relay contacts 186 and 184, contacts 166 and164, conductor 199, field winding 54, conductor 197, the secondarywinding of rate transformer 70, conductor 193, contacts 170 and 172,relay contacts 190 and 192, conductor 176 and thecollector-emitter-circuit of transistor 160 to power ground. The Nortonamplifier 152 of FIG. 6C will be responding to a logic "1" from theSolenoid Disconnecting Circuit to apply a logic "1" to the base oftransistor 156; and hence saturation level current will flow from the"run-start" contact via conductor 87 and resistor 158 and thecollector-emitter circuit of that transistor and resistor 162 to powerground. Thereupon, transistor 160 also will conduct at the saturationlevel.

Simultaneously, the Solenoid Disconnecting Circuit will be applying alogic "1" to the base of transistor 86. Consequently, when the movablecontact 83 engages the "start" contact of switch 82, current will flowfrom those contacts via solenoid coil 78 and the collector-emittercircuit of transistor 86 to power ground. The resulting flow of currentwill energize that solenoid coil and enable it to close contacts 76.Thereupon, current will flow from the positive terminal of battery 62via contacts 76, the armature 52, and shunt 72 to the negative terminalof that battery; and the magnetic field developed by that armature willcoact with the magnetic field developed by field winding 54 to causegenerator 50 to operate as a motor and rotate the crankshaft of engine56 to start that engine.

As soon as that engine starts, and causes the speed of the crankshaftthereof to exceed a predetermined value, such as three hundredrevolutions per minute, the Solenoid Disconnecting Circuit will causetransistor 86 to become non-conductive. Thereupon, relay coil 180 willbe de-energized, solenoid coil 78 will be isolated from ground,transistor 228 in FIG. 6A will become non-conductive, and diode 148 inFIG. 6C will become back biased. The Load-Sensing Circuit will cause theengine 56 to "run" at its "idling" speed; and that speed will be greaterthan the predetermined speed at which the Solenoid Disconnecting Circuitrenders transistor 86 non-conductive, de-energizes relay coil 180,isolates solenoid coil 78 from power ground, and back-biases diode 148in FIG. 6C. A typical "idling" speed would be twelve hundred revolutionsper minute. The de-energization of solenoid coil 78 will permit contacts76 to re-open; and thereafter the generator 50 will operate as agenerator rather than as a motor. The de-energization of relay coil 180will permit the setting of switch 174 to determine whether a positive ora negative polarity appears at output terminals 64 and 66. Theback-biasing of diode 148 in FIG. 6C will permit Norton amplifier 152 inFIG. 6C to respond to signals from the Hot Start Circuit, and therendering of transistor 228 in FIG. 6A non-conductive will permit theLoad-Sensing Circuit to sense and respond to the current which will passthrough shunt 72 when a welding operation is initiated.

Prior to the time the welding electrode is applied to the workpiece, nocurrent will flow through shunt 72; and hence the Current FeedbackCircuit 325 of FIG. 6A will have zero volts applied to the non-invertinginput of its operational amplifier 337. The resulting zero volts at theoutput of that operational amplifier will be applied to the Load-SensingCircuit by conductor 354; and it will cause the engine 56 to "run" atits "idling" speed as it is started, all as explained hereinbefore. Theinteraction between the magnetic field of the field winding 54 and theturns of the armature 50 will cause the generator 50 to generate avoltage between output terminals 66 and 68.

If the movable contact of mode-selecting switch 312 is in its lowerposition it will receive a signal which the Voltage Feedback Circuit 359will develop in response to the voltage across output terminals 66 and68. That signal will be a positive voltage which will be one-twentieththe "absolute" value of the voltage across output terminals 66 and 68;and that signal will be applied to the non-inverting input ofoperational amplifier 302 in FIG. 6B by conductor 313 and resistors 314and 318. That operational amplifier will respond to that signal and tothe command signal from the Direct-Reading Command Circuit to develop avoltage-modified command signal; and conductor 303 and resistor 322 willapply that modified command signal to the inverting input of Nortonamplifier 152 in FIG. 6C. The Hot Start Circuit also will be applying asignal to that inverting input; and Norton Amplifier 152 will respond tothe sum of those signals to provide an output signal which will limitthe conductivity of transistor 156, and hence of transistor 160 as well.As a result, when the operator initiates a welding operation, theinitial value of welding current will be kept low enough to keep pits orholes from being formed in the workpiece. The value of the weldingcurrent which the Hot Start Circuit permits the generator 50 to supplyto the electrode and workpiece can be adjusted by adjusting the positionof the movable contact of adjustable resistor 218.

If the movable contact of mode-selecting switch 312 is in its upperposition, that switch will apply zero volts to the non-inverting inputof operational amplifier 302 in FIG. 6B via conductor 313 and resistors314 and 318. The thumbwheel switch 233 will be applying a command signalto the inverting input of that operational amplifier; and hence thatoperational amplifier will apply a command signal to the inverting inputof Norton amplifier 152 in FIG. 6C. The Hot Start Circuit also will beapplying a signal to that inverting input; and Norton Amplifier 152 willrespond to the sum of those signals to provide an output signal whichwill limit the conductivity of transistor 156, and hence of transistor160 as well. As a result, when the operator initiates a weldingoperation, the initial value of welding current will be kept low enoughto keep pits or holes from being formed in the workpiece.

If the movable contact of mode-selecting switch 312 is in its centerposition, it will receive a synthesized summation of the voltage whichthe Current Feedback Circuit applies to potentiometer 319, of thevoltage which the Voltage Feedback Circuit applies to thatpotentiometer, and of a fixed one volt offset. That voltage will bepositive, and it will be applied to the non-inverting input ofoperational amplifier 302 in FIG. 6B by conductor 313 and resistors 314and 318. That operational amplifier will respond to that voltage and tothe command signal from the Direct-Reading Command Circuit; andconductor 303 and resistor 322 will apply a resulting modified commandsignal to the inverting input of Norton amplifier 152 in FIG. 6C in muchthe same manner, and with much the same effect, as they apply avoltage-modified command signal to that input when the movable contactof mode-selecting switch 312 is in its lower position.

When the operator initiates a welding operation, by causing theelectrode to touch the workpiece, welding current will--if switch 174 isin the "reverse" position shown by FIG. 6B--flow from brush 53 viaconductor 69, the portion 73 of the primary winding of rate transformer70, output terminal 66, the welding electrode, the workpiece, outputterminal 68, conductor 67, and shunt 72 to the brush 55. The level ofthat current will be controlled by the value of the field windingcurrent; and the Hot Start Circuit will keep that value at a desirablylow level. Consequently, the welding operation can be initiated withoutthe formation of pits and holes in the workpiece.

The IR drop across shunt 72, which will develop in response to that flowof welding current, will be applied by conductors 324 and 326 to theCurrent Feedback Circuit 325 in FIG. 6A. That circuit will respond tothat IR drop to develop a voltage of five millivolts per ampere ofwelding current at the output of its operational amplifier 337; andconductor 354 will apply that voltage to the Load-Sensing Circuit. Theconsequent energization of solenoid 482 in FIG. 6B will cause the engine56 to operate at its "full output" speed--and thereby enable generator50 to provide full welding power. Also, as welding current starts toflow, the voltage across output terminals 66 and 68 will decrease; andthe Voltage Feedback Circuit 359 of FIG. 6A will respond to thatdecreased voltage to develop a reduced voltage at the output of itsoperational amplifier 370. The application of that reduced voltage tothe non-inverting input of Norton amplifier 212 of the Hot Start Circuitin FIG. 6C will permit the output of that Norton amplifier to back biasdiode 214. Thereafter, until the electrode is moved away from theworkpiece, the Hot Start Circuit will be effectively unable to apply asignal to the inverting input of Norton amplifier 152.

If the movable contact of mode-selecting switch 312 is in its lowerposition, it will receive a signal which the Voltage Feedback Circuit359 will develop in response to the voltage across output terminals 66and 68. That signal will be applied to the noninverting input ofoperational amplifier 302 in FIG. 6B by conductor 313 and resistors 314and 318; and that operational amplifier will use that signal to modifythe command signal from the thumbwheel switch 233. As a result, theNorton amplifier 152 in FIG. 6C will set a level of conductivity fortransistors 156 and 160 which will enable generator 50 to maintain avoltage, across output terminals 66 and 68, which will match the settingof that thumbwheel switch.

If the movable contact of mode-selecting switch 312 is in its upperposition, that contact will receive a signal from the Current FeedbackCircuit 325 which corresponds to the voltage across shunt 72. Thatsignal will be a positive voltage which will be in ratio of fivemillivolts to each ampere of current flowing through that shunt. Thatsignal will be applied to the non-inverting input of operationalamplifier 302 in FIG. 6B by conductor 313 and resistors 314 and 318.That operational amplifier will respond to that signal and to thecommand signal from the Direct-Reading Command Circuit to provide acurrent-modified command signal; and conductor 303 and resistor 322 willapply that modified command signal to the inverting input of Nortonamplifier 152 in FIG. 6C. That Norton amplifier will then set levels ofconductivity, for transistors 156 and 160, which will cause the currentsupplied by armature 52 to match the level set by thumbwheel switch 233.

If the movable contact of mode-selecting switch 312 is in its centerposition, that contact will receive a signal which, in part, isdeveloped by the Voltage Feedback Circuit 359 and which, in part, isdeveloped by the Current Feedback Circuit 325. That signal will beapplied to the non-inverting input of operational amplifier 302 in FIG.6B by conductor 313 and resistors 314 and 318. That operationalamplifier will respond to that signal and to the command signal from theDirect-Reading Command Circuit to provide a command signal which ismodified by current and by voltage. Whenever the voltage across outputterminals 66 and 68 is twenty volts, the value of the welding currentwill precisely match the setting of thumbwheel switch 233.

As long as the operator continues to weld, the Current Feedback Circuit325 will supply a signal to the Load-Sensing Circuit which will enablethe latter circuit to cause solenoid 482 in FIG. 6B to operate engine 56at its "full output" speed. At that speed, the generator 50 will supplysufficient power to enable the operator to weld indefinitely. However,if the operator moves the electrode away from the workpiece for morethan twenty seconds, the Load-Sensing Circuit will de-energize solenoid482 to permit the engine 56 to operate at its "idling" speed. Also, asthe operator moves the electrode away from the workpiece, the voltageacross the output terminals 66 and 68 will increase; and the Hot StartCircuit will respond to the resulting higher-value voltage from theVoltage Feedback Circuit 359 to forward bias diode 214 in FIG. 6C andagain apply a signal to the inverting input of Norton Amplifier 152.

CONCLUSION

The rate transformer 70 performs a function which rate transformers havebeen known to perform in electric welders. Specifically, that ratetransformer responds to changes in the value of the welding current totend to change the value of the current flowing through field winding54. That change in field winding current will tend to retard the changesin the value of the welding current, and hence will stabilize the flowof welding current.

The Solenoid Disconnecting Circuit senses the speed of the crankshaft ofthe engine 56 rather than the voltage across the output terminals 66 and68; and hence that circuit provides an interlocking function which isprecisely related to the crankshaft speed. If desired, however, othervariables could be monitored to determine when the transistor 86 in FIG.6B should become non-conductive and when the logic "1" on conductors 146and 147 should change to logic "0". One of those variables is the changeof direction of current flow through the shunt 72 as the crankshaft ofengine 56 begins to rotate fast enough to cause the generator 50 tostart functioning as a generator rather than as a starting motor.Another of those variables is the rate at which air enters the intake ofthat engine. A further one of those variables is the pressure on thelubrication oil for the engine; and yet another of those variables isthe frequency of the output of generator 58. Irrespective of whatvariable is monitored to determine when the transistor 86 should becomenon-conductive and the logic "1" on conductor 146 and 147 should changeto logic "0", the Solenoid Disconnecting Circuit will keep an operatorof the welder from closing the contacts 76 while the generator 50 isproviding welding power.

The switch 174 reverses the polarity at the output terminals 66 and 68by reversing the direction of current flow through the field winding 54.The maximum value of the current which can flow through that fieldwinding is very small in comparison to the maximum value of weldingcurrent which can be supplied by the welder 10. As a result, that weldercan effect reversal of the polarity at the output terminals 64 and 66without any need of the heavy and expensive reversing switches thatfrequently are used to switch the outputs of electric welders.

If desired the potentiometer 319, of the Current-Voltage FeedbackCircuit in FIG. 6A, could be replaced by a fixed center-tapped resistor,or by a voltage divider which was made from a plurality of resistors.The resulting fixed voltage-current slope would not provide theversatility and flexibility that are provided by the potentiometer 319;but that fixed voltage-current slope would relieve the operator of theneed of making decisions as to the types of slopes to be used.

If the engine speed were to fall below the "idling" level--as it coulddo in the event an unexpected and undesired load was applied to theoutput terminals 66 and 68, the Solenoid Disconnecting Circuit wouldautomatically respond to the reduced number of pulses from the "breakerpoints" of that engine to cause Norton amplifier 136 to again apply alogic "1" to transistor 228 in FIG. 6A and to the non-inverting input ofNorton amplifier 152 in FIG. 6C, and to the base of transistor 140 inFIG. 6B. Thereupon, the Load-Sensing Circuit would develop a logic "1"at the "jumper" 460 to call for operation of the engine 56 at its"idling" speed, the Norton amplifier 152 would call for maximum fieldwinding current, relay coil 180 would make certain that the current flowthrough the field winding 54 would enable the generator 50 to act as astarting motor for that engine, and solenoid coil 78 would be connectedto power ground. As a result, the operator could, as soon as he removedthat unexpected and undesired load, shift the contacts 85 and 83 ofswitch 82 to start that engine. Similarly, when the operator "opens"switch 82, the welder will automatically pre-set the circuits which willenable actuation of that switch to re-start that engine.

The welder 10 can provide constant current welding operations such asTIG and stick welding operations. Also, that welder can provide constantvoltage welding operations such as MIG welding operations. Further, thatwelder can provide welding operations at different values ofvoltage-current slope. That welder can provide all of those weldingoperations with high degree of precision. Moreover, that welder can userelatively inexpensive parts; and, in particular, can use arelatively-inexpensive, commercially-available shunt 72.

The Welding Level Control Circuit, the Generator Field Circuit, themode-selecting Circuit and the Current Feedback Circuit or the VoltageFeedback Circuit or the Current-Voltage Feedback Circuit constitute aclosed control loop which causes the welder 10 to supply welding powerat precisely-maintained levels. As a result, that welder can providebetter welds than can engine-driven welders which do not utilize closedloop control and, instead, use the compound windings of the generatorsthereof to control the welding power which they supply. Further, becausethe generator 50 of welder 10 is not a compound-wound generator, thatwelder can, when the engine thereof is operating at its "idling" speed,supply more than thirty percent of the power which it can supply whenthat engine is operating at its "full output" speed.

In the preferred embodiment of the present invention, the armature ofgenerator 50 will be rotating at fifty-six hundred revolutions perminute when the engine 56 is operating at its "full output" speed oftwenty-four hundred revolutions per minute. At such time, the welder 10can deliver up to three hundred and seventy-five amperes at a weldingvoltage of thirty-five volts. That armature will be rotating attwenty-eight hundred revolutions per minute at no load. The engine hasan "idling" speed of twelve hundred revolutions per minute. At thattime, the welder 10 can deliver up to two hundred amperes at a weldingvoltage of about twenty volts. This means that the welder 10 can, whileengine 56 is "idling", supply up to fifty-three percent of the maximumwelding power which it can supply when that engine is operating at its"full output" speed. This is desirable, because it enables that welderto make very uniform welds--even where the command signal calls forlarge amounts of welding power.

The Hot Start Circuit, the Current Feedback Circuit, the VoltageFeedback Circuit, and the Current-Voltage Feedback Circuit, use the samelogic, namely negative logic. The fact that all of those circuits usethe same logic is desirable because it facilitates the modifying of thecommand signal supplied by the Direct-Reading Command Circuit, and alsobecause it facilitates the summing of the signal from the Hot StartCircuit with the modified command signals from amplifier 302 in FIG. 6B.

Some of the features of the present invention are as usable in electricwelders equipped with A.C. generators as they are usable in electricwelders equipped with D.C. generators. Specifically, the SolenoidDisconnecting Circuit, the Generator Field Circuit, the Welding LevelControl Circuit, the Hot Start Circuit, the Direct-Reading CommandCircuit, the Mode-Selecting Circuit, and the Load-Sensing Circuit areusable in electric welders equipped with A.C. generators.

Whereas the drawing and accompanying description have shown anddescribed a preferred embodiment of the present invention it should beapprent to those skilled in the art that various changes may be made inthe form of the invention without affecting the scope thereof.

What we claim is:
 1. An electric welder which comprises a D.C.generator, a command circuit that can develop a variable value commandsignal, a current feedback circuit, a voltage feedback circuit, acontrol circuit which receives said command signal from said commandcircuit and that can simultaneously respond to a signal from saidcurrent feedback circuit to provide a current-modified command signalwhich will be applied to the field winding of said D.C. generator, saidfield winding responding to said current-modified command signal tocause said electric welder to operate in a constant current mode, saidcontrol circuit being adapted to receive said command signal from saidcommand circuit and to simultaneously respond to a signal from saidvoltage feedback circuit to provide a voltage-modified command signalwhich will be applied to said field winding of said D.C. generator, saidfield winding responding to said voltage-modified command signal tocause said electric welder to operate in a constant voltage mode, avoltage-current feedback circuit which receives said signal from saidcurrent feedback circuit and which simultaneously receives said signalfrom said voltage feedback circuit, said control circuit being adaptedto receive said command signal from said command circuit and tosimultaneously respond to a signal from said voltage-current feedbackcircuit to provide a voltage-current-modified command signal which willbe applied to said field winding to said D.C. generator, said fieldwinding responding to said voltage-current-modified command signal tocause said electric welder to operate in a slope mode, andmode-selecting means that selectively connects said current feedbackcircuit or said voltage feedback circuit or said voltage-currentfeedback circuit to said control circuit.
 2. An electric welder whichcomprises a D.C. generator, a command circuit that can develop avariable value command signal, a current feedback circuit, a voltagefeedback circuit, a control circuit which receives said command signalfrom said command circuit and that can simultaneously respond to asignal from said current feedback circuit to provide a current-modifiedcommand signal which will be applied to the field winding of said D.C.generator, said field winding responding to said current-modifiedcommand signal to cause said electric welder to operate in a constantcurrent mode, said control circuit being adapted to receive said commandsignal from said command circuit and to simultaneously respond to asignal from said voltage feedback circuit to provide a voltage-modifiedcommand signal which will be applied to said field winding of said D.C.generator, said field winding responding to said voltage-modifiedcommand signal to cause said electric welder to operate in a constantvoltage mode, a voltage-current feedback circuit which receives saidsignal from said current feedback circuit and which simultaneouslyreceives said signal from said voltage feedback circuit, said controlcircuit being adapted to receive said command signal from said commandcircuit and to simultaneously respond to a signal from saidvoltage-current feedback circuit to provide a voltage-current-modifiedcommand signal which will be applied to said field winding of said D.C.generator, said field winding responding to saidvoltage-current-modified command signal to cause said electric welder tooperate in a slope mode, mode-selecting means that selectively connectssaid current feedback circuit or said voltage feedback circuit or saidvoltage-current feedback circuit to said control circuit, said D.C.generator being adapted to develop a positive voltage or a negativevoltage at an output thereof, means to enable said D.C. generator toselectively develop said positive voltage or said negative voltage atsaid output thereof, a subcircuit means that enables said currentfeedback circuit to provide a current feedback signal of a givenpolarity when said D.C. generator is developing said positive voltage atsaid output thereof and to provide a current feedback signal of saidgiven polarity when said D.C. generator is developing said negativevoltage at said output thereof.
 3. An electric welder which comprises aD.C. generator, a command circuit that can develop a variable valuecommand signal, a current feedback circuit, a voltage feedback circuit,a control circuit which receives said command signal from said commandcircuit and that can simultaneously respond to a signal from saidcurrent feedback circuit to provide a current-modified command signalwhich will be applied to the field winding of said D.C. generator, saidfield winding responding to said current-modified command signal tocause said electric welder to operate in a constant current mode, saidcontrol circuit being adapted to receive said command signal from saidcommand circuit and to simultaneously respond to a signal from saidvoltage feedback circuit to provide a voltage-modified command signalwhich will be applied to said field winding of said D.C. generator, saidfield winding responding to said voltage-modified command signal tocause said electric welder to operate in a constant voltage mode, avoltage-current feedback circuit which receives said signal from saidcurrent feedback circuit and which simultaneously receives said signalfrom said voltage feedback circuit, said control circuit being adaptedto receive said command signal from said command circuit and tosimultaneously respond to a signal from said voltage-current feedbackcircuit to provide a voltage-current-modified command signal which willbe applied to said field winding of said D.C. generator, said fieldwinding responding to said voltage-current-modified command signal tocause said electric welder to operate in a slope mode, mode-selectingmeans that selectively connects said current feedback circuit or saidvoltage feedback circuit or said voltage-current feedback circuit tosaid control circuit, said D.C. generator being adapted to develop apositive voltage or a negative voltage at an output thereof, and meansto enable said D.C. generator to selectively develop said positivevoltage or said negative voltage at said output thereof, a subcircuitmeans that enables said voltage feedback circuit to provide a voltagefeedback signal of a given polarity when said D.C. generator isdeveloping said positive voltage at said output thereof and to provide avoltage feedback signal of said given polarity when said D.C. generatoris developing said negative voltage at said output thereof.
 4. Anelectric welder which comprises a D.C. generator, said D.C. generatorbeing adapted to develop a positive voltage or a negative voltage at anoutput thereof, selective means to enable said D.C. generator toselectively develop said positive voltage or said negative voltage atsaid output thereof, a command circuit that can develop a variable valuecommand signal, a current feedback circuit, a subcircuit means thatenables said current feedback circuit to provide a current feedbacksignal of a given polarity when said D.C. generator is developing saidpositive voltage at said output thereof and to provide a currentfeedback signal of said given polarity when said D.C. generator isdeveloping said negative voltage at said output thereof, a voltagefeedback circuit, a subcircuit means enabling said voltage feedbackcircuit to provide a voltage feedback signal of a given polarity whensaid D.C. generator is developing said positive voltage at said outputthereof and to provide a voltage feedback signal of said given polaritywhen said D.C. generator is developing said negative voltage at saidoutput thereof, a control circuit which receives said command signalfrom said command circuit and that can simultaneously respond to asignal from said current feedback circuit to provide a current-modified,command signal which will cause said electric welder to operate in aconstant current mode, said control circuit being adapted to receivesaid command signal from said command circuit and to simultaneouslyrespond to a signal from said voltage feedback circuit to provide avoltage-modified command signal which will cause said electric welder tooperate in a constant voltage mode, and mode-selecting means thatselectively connects said current feedback circuit or said voltagefeedback circuit to said control circuit.
 5. An electric welder whichcomprises output terminals, a D.C. generator that can selectivelydevelop a positive voltage or a negative voltage at one of said outputterminals, an energizing circuit for the field winding of said D.C.generator, a polarity-controlling switch that can be set in one positionto enable said energizing circuit to cause current to flow through saidfield winding in a direction which will enable said D.C. generator todevelop said positive voltage at said one output terminal, saidpolarity-controlling switch being adapted to be set in a second positionto enable said energizing circuit to cause current to flow through saidfield winding in the opposite direction and thereby enable said D.C.generator to develop said negative voltage at said one output terminal,a command circuit that can develop a variable value command signal, acurrent feedback circuit, a control circuit which receives said commandsignal from said command circuit and that can simultaneously respond toa signal from said current feedback circuit to provide acurrent-modified command signal which will be applied to the fieldwinding of said D.C. generator, said field winding responding to saidcurrent-modified command signal to cause said electric welder to operatein a constant current mode, a subcircuit means that enables said currentfeedback circuit to provide a current feedback signal of a givenpolarity when said D.C. generator is developing said positive voltage atsaid one output terminal and to provide a current feedback signal ofsaid given polarity when said D.C. generator is developing said negativevoltage at said one output terminal.
 6. An electric welder as claimed inclaim 5 wherein a voltage feedback circuit supplies a signal to saidcontrol circuit, wherein said control circuit receives said commandsignal from said command circuit and can simultaneously respond to asignal from said voltage feedback circuit to provide a voltage-modifiedcommand signal which will be applied to the field winding of said D.C.generator, wherein said field winding responds to said voltage-modifiedcommand signal to cause said electric welder to operate in a constantvoltage mode, wherein mode-selecting means selectively connects saidcurrent feedback circuit or said voltage feedback circuit to saidcontrol circuit, wherein a subcircuit means enables said voltagefeedback circuit to provide a voltage feedback signal of said givenpolarity when said D.C. generator is developing said positive voltage atsaid one output terminal and to provide a voltage feedback signal ofsaid given polarity when said D.C. generator is developing said negativevoltage at said one output terminal.
 7. An electric welder whichcomprises output terminals, a D.C. generator that can selectivelydevelop a positive voltage or a negative voltage at one of said outputterminals, an energizing circuit for the field winding of said D.C.generator, a polarity-controlling switch that can be set in one positionto enable said energizing circuit to cause current to flow through saidfield winding in a direction which will enable said D.C. generator todevelop said positive voltage at said one output terminal, saidpolarity-controlling switch being adapted to be set in a second positionto enable said energizing circuit to cause current to flow through saidfield winding in the opposite direction and thereby enable said D.C.generator to develop said negative voltage at said one output terminal,a command circuit that can develop a variable value command signal, avoltage feedback circuit, a control circuit which receives said commandsignal from said command circuit and that can simultaneously respond toa signal from said voltage feedback circuit to provide avoltage-modified command signal which will be applied to the fieldwinding of said D.C. generator, said field winding responding to saidvoltage-modified command signal to cause said electric welder to operatein a constant voltage mode, a subcircuit means that enables said voltagefeedback circuit to provide a voltage feedback signal of a givenpolarity when said D.C. generator is developing said positive voltage atsaid one output terminal and to provide a voltage feedback signal ofsaid given polarity when said D.C. generator is developing said negativevoltage at said one output terminal.
 8. An electric welder whichcomprises output terminals, a generator that can selectively develop apositive voltage or a negative voltage at one of said output terminals,an energizing circuit for the field winding of said generator, apolarity-controlling switch that can be set in one position to enablesaid energizing circuit to cause current to flow through said fieldwinding in a direction which will enable said generator to develop saidpositive voltage at said one output terminal, said polarity-controllingswitch being adapted to be set in a second position to enable saidenergizing circuit to cause current to flow through said field windingin the opposite direction and thereby enable said generator to developsaid negative voltage at said one output terminal, a command circuitthat can develop a variable value command signal, a control circuitwhich receives said variable value command signal from said commandcircuit to provide a command signal which will determine the value ofthe current flowing through said field winding of said generator, and aHot Start circuit which selectively supplies a current-limiting signalof a fixed polarity to said control circuit which will cause saidcontrol circuit to respond to said signal from said Hot Start circuitrather than to said command signal from said command circuit and hencewill limit the value of welding current which can be supplied by saidoutput terminals during the initiation of a welding operation to a valuecorresponding to said signal from said Hot Start circuit regardless ofthe polarity of said voltage at said one output terminal and alsoregardless of the value of said command signal supplied by said commandcircuit.
 9. An electric welder as claimed in claim 8 wherein a feedbackcircuit senses the initiation of a welding operation, wherein the outputof said feedback circuit is connected to said control circuit, whereinthe output of said control circuit is connected to saidpolarity-controlling switch whereby said control circuit is connectedbetween said feedback circuit and said polarity-controlling switch,wherein said control circuit can simultaneously respond to said commandsignal from said command circuit and to a feedback signal from saidfeedback circuit to supply a feedback-modified command signal to saidcontrol circuit, and wherein the output of said Hot Start circuit isconnected between said feedback circuit and said polarity-controllingswitch.
 10. An electric welder which comprises a generator, an internalcombustion engine coupled to said generator to drive the armature ofsaid generator, a command circuit which can be set to provide apredetermined command signal, a load-sensing circuit that selectivelypermits said internal combustion engine to operate at idling speed orcauses said internal combustion engine to operate at a higher speed, anda servo system which can sense a condition of the welds to be performedby said welder to vary the current flowing through the field winding ofsaid generator and thereby cause the output of said electric welder toperform said welds in the manner called for by said command signal fromsaid command circuit, said servo system including a feedback circuitwhich senses said condition of said welds to develop a feedback signal,and means responding to said feedback signal and to said command signalto enable said current flowing through said field winding of saidgenerator to cause the output of said electric welder to perform saidwelds in the manner called for by said command signal from said commandcircuit, said load-sensing circuit being connected to said feedbackcircuit and responding to a predetermined signal from said feedbackcircuit to cause said internal combustion engine to operate at saidhigher speed, and said feedback circuit providing said predeterminedsignal whenever said output of said electric welder is performing aweld, whereby said internal combustion engine will drive said armatureat said higher speed to enable said generator to provide welding powerand said feedback signal and said command signal will provide the levelof welding power called for by said command signal from said commandcircuit.